Reading: Summary, Note & Table Completion: Question 4

Cloud seeding is a weather modification technique that attempts to increase or redirect precipitation by dispersing substances into clouds that encourage ice crystals or water droplets to form and fall as rain or snow. The technique originated in November 1946, when the American chemist Vincent Schaefer, working at a General Electric research laboratory, discovered that dropping pellets of dry ice into a supercooled cloud caused ice crystals to form almost instantly. Schaefer tested the effect in the field by releasing dry ice from an aircraft over Mount Greylock in Massachusetts, producing a light fall of snow from a cloud that had shown no sign of precipitating on its own.

Soon afterwards, a colleague of Schaefer's, the chemist Bernard Vonnegut, discovered that silver iodide could achieve a similar effect without requiring an aircraft to fly directly into a cloud, because its crystal structure closely resembles that of ice, allowing it to be dispersed from ground-based generators or released as flares at a lower altitude. Silver iodide particles act as nuclei around which supercooled water droplets freeze, and the resulting ice crystals grow heavy enough to fall as precipitation. This discovery made large-scale seeding programmes practical and considerably cheaper to run.

Assessing how well cloud seeding actually works has proved difficult, since researchers can never observe how much rain or snow would have fallen from the same cloud without intervention; controlled trials suggest that seeding may increase precipitation from a treated cloud by around fifteen percent, though the effect varies considerably with local conditions. Despite this uncertainty, China now operates the world's largest cloud-seeding programme, reportedly using it to trigger rain over Beijing in the days before the 2008 Olympic opening ceremony so that the skies above the stadium itself would remain clear. Weather modification has also been used for military purposes: during the Vietnam War, the United States ran a classified programme, Operation Popeye, to extend the monsoon season over supply routes used by opposing forces. Public disclosure of the operation contributed to the 1977 signing of an international treaty, the ENMOD Convention, which prohibits the hostile use of weather modification techniques.

Complete the summary below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

Cloud seeding attempts to increase precipitation by dispersing substances that encourage ice crystals or water droplets to form inside clouds. The technique began in (1) ____________________, when the chemist Vincent Schaefer discovered that dropping pellets of dry ice into a cloud triggered the formation of (2) ____________________. He confirmed the effect in the field by releasing dry ice over (3) ____________________ in Massachusetts, producing snow from a cloud that had not previously been precipitating. A colleague, Bernard Vonnegut, later discovered that (4) ____________________ was equally effective and, unlike dry ice, could be released from ground-based generators. Because judging the technique's effectiveness is difficult, controlled trials indicate that seeding may raise precipitation from a treated cloud by around (5) ____________________. China reportedly used cloud seeding to trigger rain over Beijing before the 2008 Olympic (6) ____________________ so that the stadium itself would stay dry. Disclosure of a covert American military programme, (7) ____________________, which extended the monsoon season during the Vietnam War, contributed to the 1977 signing of an international treaty restricting hostile weather modification.